feat(storage): notes are vault sidecar notebooks

Phase 4. Standalone notes move off SQLite into the vault, like the
PDF annotations.

- "Create notebook" makes a vault folder with a notebook.badnote.json
  (BadnoteSidecar docType 'notebook' + a title field), opened via
  SidecarRepository.
- PenNoteScreen loads/saves its strokes (page 0) + title to that
  sidecar instead of the SQLite Note model.
- note_provider lists notes from a vault scan (VaultService.scanNotes
  = folders with notebook.badnote.json and no source file); the doc
  scan still excludes them. Delete removes the folder.

PDF/slide editors unchanged; pre-existing SQLite notes migrate in
Phase 5. analyze clean, tests green.

lib/providers/search_provider.dart

@@ -2,7 +2,14 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../models/document.dart';
 import '../models/note.dart';
-import 'note_provider.dart';
+import '../services/database_service.dart';
+
+/// SQLite handle for the FTS/OCR search index. SQLite is now demoted to a
+/// rebuildable search cache (the vault sidecars are the source of truth); search
+/// is its sole remaining read path until the Phase 6 index rebuild lands.
+final databaseServiceProvider = FutureProvider<DatabaseService>((ref) async {
+  return DatabaseService.getInstance();
+});
 
 final searchQueryProvider = StateProvider<String>((ref) => '');